This invention relates to a tool holder having a built-in device for sensing, and thereby monitoring, the cutting forces applied to a cutting insert during a machining operation.
Considerable effort is currently underway to develop devices for monitoring the cutting process in order to successfully implement untended machining systems. Some of the parameters under consideration for sensing by such devices include forces, torque, deflection, temperature, vibration and power.
The device of this invention is directed to the instrumentation of a tool holder in order to sense the magnitude of the cutting force applied to a cutting insert during a machining operation and/or the values of the ratios between force components. Actually, the forces being monitored are the forces resulting from the response of the cutting tool insert to the cutting action.
Force sensors are generally termed "dynamometers". Dynamometers for such force sensing incorporate sensors on the tool holder and the tool holder is mounted in a housing. Wire strain gage dynamometers have been used in this manner, but these are not very sensitive due to low gage factor (i.e. ability to convert strain signal into voltage signal) of about 2. Consequently, when wire strain gages have been used it has been necessary to use tool holders having a long overhang with the gage section (i.e. where the strain gages are mounted) having the smallest cross-section possible in a workable system. Such an arrangement has the inherent disadvantage of reducing the stiffness of the tool holder. Further, the response frequency of this type of dynamometer is also low. Semiconductor strain gage dynamometers are more sensitive (i.e. gage factor of approximately 120) but the response frequencies of these dynamometers are still low (i.e. a few hundred Hertz). Piezoelectric dynamometers (e.g. the Kistler piezoelectric 3-component lathe dynamometer Model 9263) have higher sensitivity as well as higher response frequency. However, the response frequency of commercial lathe dynamometers is still low since the sensing units are located in a housing, which imposes a heavy mass on top of the load cells. In addition to having low response frequency, the sensing system with its housing is bulky and inconvenient to mount on a machine tool turret.
In an attempt to improve force sensing capability, a paper presented by Lindstrom and Lindberg presented at the 24th International Machine Tool Design and Research Conference (Aug. 31 to Sept. 1, 1983) at Manchester, England describes a force monitoring arrangement in which a one-component force sensor using a piezoelectric element was mounted in the tool holder directly under the cutting tool insert. In this arrangement the normal component of the cutting force acted at one corner of the load cell. Such a force application to any load cell will result in the imposition of a bending moment to the load cell, a stress, which piezoelectric load cells are incapable of withstanding to any significant degree.